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The World Health Organization and Thimerosal

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Qs and As on thiomersal
July 1999

  1. What is thiomersal?

    Thiomersal (also known as thimerosal or mercurothiolate) is a mercury-containing compound used in trace amounts to prevent bacteria and other organisms from contaminating vaccines, especially in opened multi-dose vials.

  2. Is thiomersal a new component of vaccines?

    No, it is not new. Thiomersal has been used for about 60 years in the manufacture of certain vaccines. There have been no reports of adverse events due to thiomersal except for very rare reports of sensitization (skin rashes).

  3. What is WHO doing about thiomersal in vaccines?

    - WHO is working to bring about a move away from the use of thiomersal in vaccines when an effective alternative becomes available, and is working with national regulatory authorities and manufacturers to bring this about.

    - WHO is explaining that the risk to unvaccinated children of death and complications from vaccine-preventable diseases in is real and enormous. The risk from side effects of thiomersal is theoretical, uncertain and, at most, extremely small.

    - WHO is aiming to replace thiomersal with other preservatives in the long term.

    - WHO is examining possibilities for combining several vaccines together in one vial. In this way, the same quantity of thiomersal can act as a preservative for several vaccines at once, thereby reducing to an absolute minimum the amount of mercury given to each child.

  4. When was it determined that high levels of thiomersal content in vaccines is a potential problem?

    The level of mercury contained in three doses of DTP, hepatitis B and Hib vaccines (9 doses in all) does not exceed the WHO limit for mercury intake over the first 12 months of life. It was noticed by officials in the United States recently that infant immunizations could theoretically raise mercury levels above those set by certain US regulatory bodies.

  5. Do all vaccines contain thiomersal?

    No, not all vaccines contain thiomersal. None of the "live" vaccines contain it: measles, MMR, oral polio, yellow fever, BCG. Many of the other vaccines in single-dose vials do not contain it either. Injectable polio vaccine is also free from thiomersal. Triple vaccine (DTP) may not contain thiomersal if the pertussis component is the acellular vaccine.

  6. Is there a replacement for thiomersal?

    There are a number of other chemicals such as 2-phenoxyethanol used as vaccine preservatives. But none does the job as well as thiomersal. Even if others are found appropriate, it will take time to introduce them. Replacing the preservative in a vaccine means it becomes a "new product" for licensing purposes and has to go through the pathway to be tested and fully licensed. It is exactly this pathway which ensures vaccines are as safe as possible.

  7. Do vaccine package inserts contain information about whether thiomersal is in the vaccine?

    They may not contain this information. Inserts of vaccines procured by WHO and UNICEF already contain this information.

  8. Who is most at risk from mercury?

    Mercury is not good for anyone. But those most at risk are unborn babies and newborn babies. The brains of these little ones are the organs most at risk from mercury. The type of mercury in thiomersal is not the same type or form as that causing poisonings during industrial accidents. It is not as toxic because it is bound to other chemicals in the thiomersal molecule and is not free to react with the human body in the same way. The amount of mercury in vaccines is minute compared with the amount generally involved in accidents.

  9. What should parents do (vaccinate or not vaccinate with thiomersal-containing vaccines)?

    For children in industrialized and developing countries alike, the risk of death and complications from vaccine-preventable diseases is real and enormous. The risk from side effects of thiomersal is theoretical, uncertain and, at most, extremely small. The best advice to parents is to continue having their children vaccinated. The balance of risk is clearly in favour of continuing to vaccinate. Ensuring the quality and safety of vaccines is a continuous process for WHO, national regulatory authorities and manufacturers.

  10. What proof do parents have that vaccinating their children is safe? What scientific information do you have to backup your recommendation to vaccinate?

    Vaccines containing thiomersal have been used for 60 years without bad effects being noted from thiomersal. There is no information showing that thiomersal in vaccines is dangerous. The discussions currently are about theoretical risks.

  11. What should doctors and vaccinators tell worried parents?

    Parents should be told as much as they want to know and are able to understand. The issues are very complex and most parents do not want to know all the science – they generally want to be reassured something is safe or not safe by a person they trust.

  12. Can all vaccines be made thiomersal-free? If so, how quickly?

    Most vaccines could be made thiomersal-free quite quickly, but they would not contain a preservative. It is not safe to use multi-dose vials of certain vaccines without some form of preservative. One solution would be to use single-dose vials, but this solution is very expensive and not always technically possible. If a new preservative were to be used, the product would have to be relicensed, taking a long time. Equally if thiomersal was removed from a vaccine, it would have to be re-licensed as well.

  13. Are children who have been given vaccines with thiomersal at risk for adverse side effects? What are the possible side-effects? What kind of treatment can a child receive?

    There is a potential risk of a skin sensitization producing a rash with any mercury-containing products. The amount of thiomersal in vaccines is very small, and the risk of adverse reactions are remote and theoretical. However, high levels of mercury in pregnant mothers may result in brain damage to the unborn baby. This has happened rarely in the past when, for instance, grain contaminated by mercury was eaten. Children who have received thiomersal products do not need treatment.

  14. How many children in the world have been given more than the recommended amount of thiomersal through vaccines?

    WHO estimates that no child receives more than the WHO recommended dose of mercury from vaccines alone in the first year of life. Some communities where a lot of fish is eaten (fish may contain a high mercury content) may raise their mercury levels above the recommended levels.

  15. In light of this information, would you vaccinate your own children right now with thiomersal-containing vaccines?

    Yes. I was vaccinated with triple vaccine as a child and my own children have been given triple vaccine and hepatitis B vaccine. Knowing about thiomersal would not change those decisions for me.

  16. Should other countries follow the advice given to the American public regarding hepatitis B vaccination of the newborn?

    The advice to the American public makes good sense for the United States. But for developing countries where the risk of hepatitis B being transmitted from mother to baby is very high, WHO current recommendations should continue. WHO does not recommend screening before vaccination in national vaccination schedules. There are millions of new cases of hepatitis B each year, resulting in thousands of deaths in adults annually. The risk from the disease is huge, the risk from the presence of thiomersal in the vaccine is theoretical, uncertain, and at most, extremely small.

  17. Will there be a global shortage of vaccines now?

    WHO, UNICEF and other public health authorities all consider there is no reason to stop using current vaccines. If public opinion accepts this, there will not be a problem. If opinion-leaders create a panic situation, the public might refuse to use thiomersal-containing vaccines. Manufacturers would then not be able to turn to alternative products to meet the needs of the world's children.

  18. Will children in the developing world suffer?
    WHO and UNICEF are fully committed to avoiding any hardship, suffering or disadvantage for children in the developing world as a result of the thiomersal discussion. The children have enough to put up with already.




Note to the press No 18

9 July 1999


The World Health Orgainzation (WHO) underlines the importance of continuing to use currently available children's vaccines. The Organization supports a statement made on the 7th July 1999 by the American Academy of Pediatrics and the United States Public Health Service regarding prospective phasing out of thiomersal – a chemical compound containing mercury used in trace amounts as a vaccine preservative. Thiomersal (also known as thimerosal or mercurothiolate) has been used since the 1930s to prevent bacteria and other organisms from contaminating vaccines, especially in opened multi-dose vials.

WHO will continue recommending procurement of vaccines such as triple vaccine (DTP), diphtheria and tetanus (DT), tetanus toxoid (TT) and hepatitis B that contain the preservative. These vaccines have been used safely throughout the world for over 60 years, helping to save many millions of children's lives.

For children in industrialized and developing countries alike, the risk of death and complications from vaccine-preventable diseases to unvaccinated children is real and enormous. "We support the continued use of current vaccines that protect the lives of millions of children each year," says Dr Scholtz, WHO's Executive Director of Health Technology and Pharmaceuticals.

The US report makes recommendations to avoid thiomersal-containing hepatitis B vaccine for certain newborns, including the screening of pregnant mothers for the blood marker of the liver infection. WHO underlines this is not an option for developing countries due to practical constraints and the high cost. Instead, the Organization promotes the development of a single-dose presentation of hepatitis B vaccine that will not require a preservative. This "Uniject" device is a disposable plastic pouch-and-needle, ideal for delivering the newborn dose in developing countries. It is presently undergoing field trials.

WHO will continue working with industry and national regulatory authorities to eliminate thiomersal from vaccines as soon as possible.




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